If you are a smoker, you are going to have a constant battle trying to keep your teeth white. If you are that worried about a white smile, you are going to need to quit or at least cut down in the amount that you are smoking. It will be a never ending battle if you continue to smoke heavily.
As you age, teeth will naturally yellow so it is important to have preventative care and whitening done. Maintain proper dental visits and cleanings as well as your regular brushing habits. Minimize sodas, teas, coffees and avoid excesses of sticky foods. These things will prolong the natural white color of your teeth.
You are going to damage your teeth in a way that cannot be repaired if you over treat your teeth with teeth whitening gels or pastes. Only use them as they are directed on the label of the brand that you are using. You will avoid doing damage to your teeth when trying to get them white.